This fixes various allocator methods to properly propagate {nullptr} to
callers without accidentally dereferencing it. We also disable one test
case for stress mode as it runs out of memory due to inlining limits
being lifted in the stress mode.

R=bmeurer@chromium.org
TEST=mjsunit/array-natives-elements
BUG=v8:6061

Change-Id: Id0a7b826a8612d00b4f4ae8aa0bea011c50890ca
Reviewed-on: https://chromium-review.googlesource.com/451365Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43672}